# problem in DNS in a pipe

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October 9, 2012, 05:51
problem in DNS in a pipe
#1
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Hassan Nemati
Join Date: Apr 2012
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Hi guys
I am running a DNS code in a pipe. For laminar flow I have a problem.
as you see in 2d plot of radial velocity at inlet, there are some strange number of velocity. it should be zero but it is not.
I will appreciate any help.
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October 9, 2012, 06:20
#2
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
 Originally Posted by hnemati Hi guys I am running a DNS code in a pipe. For laminar flow I have a problem. as you see in 2d plot of radial velocity at inlet, there are some strange number of velocity. it should be zero but it is not. I will appreciate any help.
You should address more details ... Re number, number of cells, inflow type ...
are you solving incompressible flows? did you check the divergence-free constraint in each cell? Are you sure of the numerical stability contraints?

From the figure it seems that the radial velocity accounts for the wall and the incipient boundary layer in a very low Re flow with plug velocity inlet, but I may be wrog ...

October 9, 2012, 07:58
#3
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Hassan Nemati
Join Date: Apr 2012
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Quote:
 Originally Posted by FMDenaro You should address more details ... Re number, number of cells, inflow type ... are you solving incompressible flows? did you check the divergence-free constraint in each cell? Are you sure of the numerical stability contraints? From the figure it seems that the radial velocity accounts for the wall and the incipient boundary layer in a very low Re flow with plug velocity inlet, but I may be wrog ...
Hi
the Re=60 based on friction velocity. number of mesh is: r=30 theta=16, z=96.
Yes this is for incompressible flow. the divergence is Ok. I am using the profile ((Re/4.)*(1.- 4.*(rp(i))**2.)) for inlet which is analytitical solution. I put radial and tangential velocity 0 at z=0 .

October 9, 2012, 09:46
#4
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
 Originally Posted by hnemati Hi I am so grateful for your answer. the Re=60 based on friction velocity. number of mesh is: r=30 theta=16, z=96. Yes this is for incompressible flow. the divergence is Ok. I am using the profile ((Re/4.)*(1.- 4.*(rp(i))**2.)) for inlet which is analytitical solution. I put radial and tangential velocity 0 at z=0 .
Ok, are you sure you get a full steady state?
After you have to refine the grid and see what happens, usually the small value in the normal-to-wall velocity component can be controlled. See also your threshold value in the pressure solver, can be to high.
After that, you can check further by changing the inflow/outflow condition in a periodic condition, this way you can check the inner solution.

October 22, 2012, 12:10
how strange the velocity is? Can you email me and we can discuss furtherly.
#5
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kuihe
Join Date: Oct 2012
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Quote:
 Originally Posted by hnemati Hi guys I am running a DNS code in a pipe. For laminar flow I have a problem. as you see in 2d plot of radial velocity at inlet, there are some strange number of velocity. it should be zero but it is not. I will appreciate any help.